Class D audio amplifiers are a well-known type of audio power amplifier which is generally recognized to provide energy efficient audio drive of a loudspeaker load by switching a pulse width modulated (PWM) or pulse density modulated (PDM) signal across the loudspeaker load. Class D audio amplifiers typically comprises an H-bridge driver with a pair of output terminals coupled to respective sides or terminals of the loudspeaker load to apply an oppositely phased pulse width modulated or pulse density modulated audio signals across the loudspeaker. Several modulation schemes for pulse width modulated audio signals have been utilized in prior art PWM based class D amplifiers. In so-called AD modulation, the pulse width modulated audio signal at each output terminal or node of the H-bridge is switched between, or toggles between, two different levels in opposite phase. The two different levels typically correspond to the upper and lower power supply rails, respectively, such as the positive and negative DC supply rails of the class D amplifier. In so-called BD modulation, the pulse width modulated signal across the loudspeaker load is alternatingly switched between three levels of which two levels correspond to the above-mentioned upper and lower DC power supply rails and the third level is zero level that is obtained by simultaneously pulling both terminals of the loudspeaker load to one of the DC power supply rails. In multi-level PWM modulation as described in the applicant's co-pending patent application PCT/EP2011/068873, a third supply voltage level, often a set to a mid-supply level between the positive and negative DC supply rails, is applied to output node(s) of the output driver such that for example a 3-level or 5-level pulse width modulated signal can be applied across the loudspeaker load by an appropriately configured output driver.
The present inventor has obtained significant performance improvements in Class D amplifiers by adjusting the modulation frequency of the pulse width modulator according to specific characteristics of the audio input signal. The modulation frequency adjustment is beneficial for several reasons such as reduced EMI emission, reduced power loss in output switching devices etc. However, frequency response characteristics of loop filters of class D audio amplifiers are ordinarily designed or selected such that the feedback loop remains just stable, within a required safety margin due to component spread and production spread, at full output power and maximum setting of the modulation frequency. This loop filter design is advantageous in order to suppress distortion and noise generating mechanisms introduced by non-ideal behaviour of circuitry and components of the class D amplifier within the feedback loop. However, the just stable design of the loop filter at the maximum setting of the adjustable modulation implies the feedback loop tends to become unstable if a lower modulation frequency than the maximum frequency setting is chosen. Such a lower modulation frequency would otherwise be advantageous for example to save power as described above. Hence, it is advantageous to tailor or adapt the frequency response characteristics of the adjustable loop filter to a current setting of the modulation frequency, and optionally load power level, to obtain the highest possible loop gain of the feedback loop throughout the audio bandwidth (20 Hz-20 kHz) within loop stability constraints for specific settings of the modulation frequency of the pulse width modulator.
Accordingly, class D audio amplifiers with reduced levels of EMI noise are highly desirable. Likewise, class D amplifiers with improved power efficiency, especially at low or small audio input signals levels, are also highly advantageous. Finally, it is desirable to decrease the size of the external load inductors and load capacitors to provide more compact, power efficient, reliable and less costly Class D amplification solutions for consumer and other types of audio products.